1. Field of the Invention
The present invention relates to three dimensional computer graphics (CG), and, in particular, to an image generating system for displaying a three dimensional (3D) model in a cel animation style.
2. Related Art Statement
Owing to the recent improvement in hardware performance, even a consumer video game machine can generate images of three dimensional CG (Computer Graphics) in real time, realizing a three dimensional CG animation.
Further, according to contents of a game or the like, it is sometimes desired to display such a three dimensional CG animation in a conventional cel animation style. In a cel animation, an image is generated by drawing lines and painting the inside of each drawing, and, as a result, a character or the like has contours. However, according to ordinary three dimensional CC, contours are not drawn. Thus, to realize display in a cel animation style, contours should be drawn in a 3D model.
A 3D model is made of, for example, a plurality of surfaces. To draw contours in such a 3D model, it is necessary to draw a line between specific surfaces, namely between a surface that faces the front side and a surface that faces the back side, seen from a viewpoint.